1. Field of the Invention
The present invention relates to line printers, and more particularly to print hammer mechanisms for controlling the operation of a plurality of resilient elongated hammer elements mounted within a reciprocating hammer bank and having dot matrix impacting elements mounted thereon.
2. History of the Prior Art
It is known to provide in a dot matrix line printer a reciprocating shuttle containing a hammer bank in which a plurality of elongated, resilient, generally parallel hammer elements having dot impacting tips at the free ends thereof are selectively released from retracted positions so as to impact an ink ribbon against a platen supported print paper as the shuttle reciprocates relative to the print paper. Such an arrangement is shown in U.S. Pat. No. 3,941,051 of Barrus et al, issued Mar. 2, 1976 and commonly assigned with the present application. In the Barrus et al patent, the hammer bank employs a print hammer mechanism which forms a generally C-shaped magnetic circuit between the opposite fixed and free ends of the hammer elements. The magnetic circuits include a common permanent magnet to which the hammer elements are coupled at their fixed ends, a common magnetic return path coupled to the permanent magnet opposite the hammer elements and a plurality of pole pieces, each of which extends outwardly from the magnetic return path so as to terminate in a pole tip facing the free end of the hammer element. Flux from the permanent magnet normally pulls the hammer element out of a neutral position and into a spring-loaded retract position against the pole piece. Each time a coil surrounding the pole piece is momentarily energized, the attracting force of the permanent magnet is overcome long enough to release the hammer element from the retract position and send it flying in the direction of the ink ribbon and print paper. Following impacting of the dot printing tip against the ribbon and paper, the hammer element rebounds back into the spring-loaded retract position in preparation for the next energization of the coil.
The coils themselves are individually wound on bobbins with each bobbin surrounding a different pole piece. Each bobbin mounted coil must typically be provided with a finned heat dissipating element as shown, for example, in U.S. Pat. No. 4,033,255 of Kleist et al to provide adequate dissipation of heat generated by the coil.
The print hammer mechanism disclosed in Barrus et al U.S. Pat. No. 3,941,051 has been found to function effectively and efficiently for practically all applications of the line printer. However, there may be occasions where improvements in performance are desired. Such occasions may arise, for example, where space limitations within the line printer or within the hammer bank dictate a reduction in the width or thickness or both of the hammer elements. Such conditions may require an increase in the amount of magnetic energy, or conversely an increase in the efficiency of the magnetic circuit such that the magnetic flux available is more efficiently utilized. It is also desirable to avoid use of finned heat dissipating elements with the coils wherever possible.
Accordingly, it is an object of the invention to provide an improved print hammer mechanism.
It is a further object of the invention to provide an improved print hammer mechanism providing better hammer element release and retraction for a given amount of magnetic flux.
It is a still further object of the invention to provide an improved print hammer mechanism in which the resonant frequency of the hammer element can be increased for a given amount of magnetic flux.
It is a still further object of the invention to provide an improved print hammer mechanism the force-displacement characteristics of which can be varied so as to reduce the magnetic energy needed for hammer element release, to improve hammer element retraction and to enable other magnetic characteristics of the mechanism to be varied and generally improved upon.
It is a still further object of the invention to provide an improved print hammer mechanism in which adequate dissipation of heat from the coils is accomplished without finned heat dissipating elements or similar elements being mounted on the coils.